Control of molybdenum during liquidliquid extraction of uranium using amine extractants



March 26, 1963 J. L. DROBNICK ETAL 3,083,0761/ CONTROL OF MOLYBDENUM DURING LIQUID-LIQUID EXTRACTION 0F URANIUM USING AMINE EXTRACTANTS Filed June 5, 1959 URANIUM 01E ACID ACID LEACH (D AMINE REAGENT AMINE EXTRACTION 4- SPENT AQUEOUS SOLUTION To URANIUM PREGNANT WASTE AMINE REAGENT MOLYBDENUM BLEED REDUCTION AQUEOUS STRIPPING REAGENT STRIPPING C AGuEous SOLUTION TO URANIUM RECOVERY INVENTORS JAMES L. DROBNICK CLIFFORD J. LEWIS ATTORNEY" United States Patent CONTROL OF MOLYBDENUM DURING LIQUID- LIQUID EXTRACTION 0F URANIUM USING AMINE EXTRACTANTS James L. Drobnick, Golden, and Clifford J. Lewis, Lakewood, Colo., assignors to General Mills, Inc., a corporation of Delaware Filed June 5, 1959, Ser. No. 818,307 8 Claims. (Cl. 23-145) The present invention involves a process of extracting uranium from its ores involving the use of liquid amine extractants, and particularly the control of molybdenum during such extraction process. In such processes, molybdenum which frequently accompanies uranium ores, has heretofore caused some operating difliculty in that it precipitates as a complex with the amine extractant in the process and interferes with the efiicient separation of the uranium.

In the typical acidic uranium extraction process the uranium ore is leached with an aqueous acid solution such as sulfuric acid in order to produce a solution containing the uranium values. The uranium values may be extracted from this aqeous solution by an nonaqueous amine solution and the uranium values are recovered from the amine solution by an aqueous stripping solution. Molybdenum which ordinarily accompanies uranium in ores of this type is likewise soluble in the acid leaching solution and is likewise soluble in the amine extractant. The molybdenum however appears to remain in the amine extractant and does not strip therefrom by the aqueous solution commonly employed for stripping the uranium. Accordingly the molybdenum tends to build up in the amine extractant and it is necessary to bleed off a stream of this amine extractant for the purpose of stripping molybdenum from it. It is found that in ordinary operations the moly-bdneum seems to form some sort of complex with the amine extractant, which complex is insoluble and appears at the interface between the organic phase containing the amine extractant and the aqueous stripping solution for the uranium. While the presence of these interfacial solids in the stripping circuit does not completely stop the operation these solids must eventually be removed along the circuit. Moreover in the removal of these solids along the circuit, entrained amine reagent is lost, thus increasing reagent costs.

It has been discovered that the control of the formation of the insoluble molybdenum amine complex can be exercised by controlling the valence state of the molybdenum. In the past the molybdenum has been maintained in the molybdate form by use of an oxidizing agent which was found to prevent the formation of the interfacial solids. However when the molybdenum content is high the use of an oxidizing agent has been found to be relatively ineffective. It has now been found that the use of a reducing agent controls the valence of the molybdenum so that the interfacial solids do not appear inthe amine extractant even when the molybdenum content is very high. In addition control of the valence state of the molybdenum through the use of a reducing agent increases the tolerance level of tertiary amines for molybdenum to a point where the tolerance level is equal to or greater than that of the secondary amines. Thus the use of a tertiary amine in the circuit can be employed 3,083,076 Patented Mar. 26, 1963 which thereby takes advantage of the greater capacity of this type amine for uranium as compared with the capacity of the secondary amines. Where the molybdenum level in the uranium solution is high, past practice utilizing an oxidizing agent dictates the use of secondary amines as such amines can tolerate a considerable amount of molybdenum before formation of the interfacial solids. Accordingly with the use of a reducing agent the tertiary amines can be used advantageously.

It is therefore an object of the present invention to provide a novel process for the extraction of uranium from its ores involving the use of amine extractants in which the appearance of green interfacial solids in the amine extractant is prevented.

It is also an object of the present invention to provide a process for the extraction of uranium from its ores involving the use of amine extractants in which the molybdenum content is high.

Another object of the present invention is to provide such a process in which amines having a greater capacity for uranium can be used without the appearance of green interfacial solids in an amine extractant.

The invention will be described with reference to the drawing which is a flow sheet of a typical uranium extraction process. The finely divided uranium ore is leached with a strong mineral acid such as sulfuric or hydrochloric. This leaching step dissolves the uranium, molybdenum, iron and aluminum principally and the separation process involves the isolation of the uranium from this solution. The acid solution is then subjected to a liquid-liquid extraction with any of a variety of amines which will be described in more detail hereinafter. The organic phase amine extractant is immiscible with the aqueous phase and the uranium is transferred from the aqueous phase into the organic phase. The process involved is essentially an ion exchange reaction. The amines represent an anion exchange reagent. Since uranium in its acid leached solutions exists in an equilibrium involving uranium in the form of anions and cations it can be solvent extracted by the amines as anion exchange reagents, the removal of uranium anions causing the equilibrium to shift until all of the uranium is removed. Molybdenum also exhibits this equilibrium and accordingly, accompanies the uranium in the amine extraction. Other metals such as iron and aluminum which are present in the acid leach exist only as cations in the pH range involved and accordingly, these metals are not picked up by the amine reagents.

In the conventional prior art processes the uraniumpregnant amine reagent is sent directly to a stripping operation in which it is contacted with an aqueous solution generally consisting of sodium chloride or sodium chloride acidified with sulfuric acid. These solutions readily strip uranium from the organic phase. Molybdenum ions however form strong associations with amine extractants and are not readily removed by such stripping agents as sodium chloride or acidified sodium chloride solutions. An aqueous solution of sodium carbonate is effective for stripping molybdenum from the amine reagents. Accordingly a bleed stream technique has come into practice wherein the conventional sodium chloride solution is used as the primary uranium stripping solution and a small bleed stream of the uranium barren organic phase is routed through a sodium carbonate stripping solution which removes the molybdenum. The

bleed stream of amine reagent is then Combined with the main stream of reagent for recycling through the operation. In this manner the molybdenum concentration in the organic phase is held at a definite level. However some molybdenum amine complex does remain in the organic phase and can build up to cause the difficulty referred to previously, namely, the formation of a precipitate which deposits at the interface of the organic and aqueous phases in the stripping cells. This gives rise to mechanical difficulties in a circuit which was designed to be completely liquid at all times.

In the past, two solutions have been proposed for the prevention of the formation of the interfacial solids. The first of these is the incorporation of water immiscible additives to the organic phase which render the molybdenum amine precipitate soluble. This is not a complete solution to the problem. These additives are relatively expensive and would have to be used in considerable quantity in order to accomplish the desired end. They thus decrease the extractive potential of the organic phase or at least increase the volume of organic phase handled. Moreover the amount of additives required could well lead to emulsion difficulties as well as to an impractical rate of phase disengagement. The second of these has been the introduction of an oxidizing step between the extracting and stripping steps. By this step the molybdenum is apparently maintained in the molybdate form, thus preventing the formation of a complex which seems to be based on the presence of molybdenum in the molybdyl form. As such this method is effective when the molybdenum concentrations are fairly low. In addition amine extractants which have a greater capability for the uranium have not been able to be used in the process because of their low tolerance for molybdenum which results in formation of the interfacial solids.

As was indicated previously it has now been discovered that formation of an undesirable molybdenum amine complex can be prevented by the introduction of a reducing step between the extraction and the stripping steps. By the use of the reducing step it has also been found that amine extractants which have a greater capacity for the uranium can be used without the formation of the interfacial solids. Although it is preferred to introduce the reducing step between the extraction and stripping step, it is also possible to introduce the reducing agent directly into the first stripping cell. The use of the reducing agents apparently reduces the molybdenum to a valence state which also prevents the precipitation of the undesirable molybdenum amine complex. Possibly it is the molybdyl molybdenum which causes the precipitation or it may be a combination of molybdyl molybdenum with some other valence state of molybdenum, The use of an oxidizing agent such as hydrogen peroxide apparently prevents the combination by converting the molybdyl form upwards or by changing the molecular configuration. The use of reducing agents appears to achieve the same effect by converting the valence state downwards.

While the invention is not to be limited thereto, the following theoretical explanation might account for the reason that oxidation is not effective in the presence of substantial molybdyl molybdenum while the use of a reducing agent is effective. It is known that molybdenum in acidic uranium-pregnant solutions exists in an equilibrium involving both molybdenum anions and cations. This is apparent, since otherwise molybdenum could not be extracted by either anion extractants such as amines r cation extractants such as organo-phosphates. It is believed, therefore, that molybdenum exists in an equilibrium involving various valence states from molybdate to the molybdenum cation. Accordingly, if molybdyl molybdenum is the cause of the interfacial solids, it requires a considerable amount of driving force to convert this all to the molybdate form, since this means 100% equilibrium shift in one direction. "On the other hand less driving force should be required to substantially convert molybdyl molybdenum to the lower valence state since this does not require that the equilibrium be shifted completely to only one valence state; in other words, there is more than one valence state toward which molybdyl molybdenum may move under a reducing influence while there is but one higher valence state which can be realized under an oxidizing influence.

A variety of reducing agents may be useful for accomplishing the reduction step. A simple and inexpensive means for accomplishing this is by introduction of an aqueous sodium sulfide solution into the process between the extraction and the stripping steps. The uranium pregnant amine reagent may be scrubbed with a dilute aqueous solution of sodium sulfide such as one containing approximately 1% by weight of sodium sulfide. In some instances the reagents could be fed dry or in various other water concentrations. The important item is to add sufficient reagent to control the valence state of molybdenum and therefore the quantity of reagent is chiefly determined by the amount of molybdenum present. The sodium sulfide is apparently decomposed thus allowing the surfur component of the reagent to exert the necessary reducing action on the molybdenum present in the uranium loaded organic phase. The end product of the sulfur reagent decomposition reports into the aqueous phase thus following the aqueous product concentrate,

Following the stripping of the uranium pregnant amine reagent, the amine reagent may be recycled with only a small bleed stream being taken off for removal of molybdenum. Generally a 40% volume bleed stream or less may be used for this purpose. Bleed streams down to as little as 10% of the amine reagent being recycled have been found effective for maintaining the molybdenum level low enough for numerous cycles of the organic circuit. Generally, however, bleed streams in the range of 20% of the volume of the recycle amine reagent are preferred.

It will be appreciated that the above description is with reference to the drawing which is a flow sheet of a simplified extraction system. It will also be appreciated that in commercial practice the process would most efliciently be conducted on a system involving multiple stage extraction and stripping in which countercurrent flow would generally be employed. Those skilled in the art will readily appreciate the manipulative steps which may be employed to utilize the herein described process on a commercial scale.

EXAMPLE I 1.40 g. per liter. 0.147 g. per liter. E m f 375 mv.

The amine employed was a tertiary alkyl amine in which the alkyl groups were straight chain hydrocarbon groups containing principally 8 and 10 carbon atoms with a minor amount of 12 carbon atom alkyl groups. These alkyl groups were derived from the mixed C C and C acids of coconut oil. The amine reagent was composed of volume pervent kerosene, 5 volume percent isodecanol and 5 volume percent of the above-mentioned tertiary alkyl amine. The extraction was conducted in four stages with a volume ratio of uraniumpregnant liquor to the organic extraction phase of 2.8, one stage of a sodium sulfide scrub of the uraniumpregnant amine reagent with a 0.69 percent by weight of sodium sulfide solution having a pH of 7.0, and four stages of stripping with a 1.5 normal sodium chloride solution. The stripping circuit was operated with a volume ratio of organic amine phase to acid stripping reagent of 9.0. The process was conducted continuously through a series of cycles of organic flow and the bleed stream was varied from 0 to so that the molybdenum metal in the recycle organic was allowed to build up" to approximately 2.0 grams Mo per liter. The stripping of the molybdenum was accomplished by means of a by weight solution of the sodium carbonate in water. The results are indicated in the following table:

The data in column 2 show that the sulfide scrub does not significantly alter the stripping properties of U 0 from the loaded organic phase. Without the sodium sulfide scrub the stripping cycle could not tolerate more than 0.4 gram per liter of M0 in the amine reagent without forming an insoluble third phase. The data in column 3 show that the Mo concentration can increase to more than 1.8 grams per liter without the third phase forming, when a sodium sulfide scrub is used.

EXAMPLE II In this example sodium thiosulfate was used as the reducing agent and a 40% volume bleed stream was used for the removal of molybdenum. In all other respects the run was performed as in Example I. A 2% aqueous solution of sodium thiosulfate was used which was added at the rate of 0.2 pound of sodium thiosulfate per pound of U 0 The circuit ran very well and operated smoothly for two cycles when sodium thiosulfate consumption was cut to 0.05 pound per pound of U 0 The eighth cycle conditions were as follows:

G. per liter U 0 in loaded organic 5.2 U 0 in stripped organic 0.025 U 0 in aqueous barren 0.007 M0 in loaded organic 0.97

The data show that the sodium thiosulfate scrub not only was not interfering with the extraction efficiency of the organic phase but also that the uranium was being effectively stripped from the organic phase while at the same time molybdenum was progressively building up in the organic phase.

In the place of the specific amine mentioned above any of a variety of amines may be used which are capable of extracting uranium from an acid leach liquor. These amines are generally aliphatic in character although they may be partially aromatic. In general, the amines are either secondary or tertiary amines, particularly those containing aliphatic hydrocarbon groups from approximately 8 to approximately 22 carbon atoms. The tertiary amines are preferred as they have a greater capacity for the uranium. These aliphatic hydrocarbon groups may be straight chained, saturated or unsaturated. In place of these straight chain aliphatic hydrocarbon groups the amines may contain highly branched chain aliphatic hydrocarbon groups which are principally derived from olefinic sources. In addition it is possible to use amines containing one or more branched chain alkyl groups and/or one or more straight chain alkyl groups. Typical amines which may be used by this purpose are the following:

in which R is an aliphatic hydrocarbon group having a tertiary carbon atom at the point at which it is attached to the nitrogen atom. The substituents attached to this tertiary carbon atom are alkyl groups totaling from 11 to 14 carbon atoms in the three alkyl groups. R is a C H group. A product of this type is available on the market and is sold as Amberlite .LA-2.

\NH R5 in which R is as above-described and R is the group CH3 CH3 o11 b-oHr( )-0HiCH=oHoHr A product of this type is available on the market and is sold as Amberlite LA-l.

(3) Tri-isooctylamine. (4) The compound bis(1-isobutyl-3,S-dimethylhexyl) amine having the formula CH1 CH3 0H OH OHOHzCHCHflJHCHH JHCH s'IH CHaCOCHzCHCHzHCHzCHCHa H3 H3 (5H3 (5) Di(3,5,7-trimethyloctyl)amine having the formula CH3 CH3 CH3 OH HOHzfiOHzHOHzoHn \NH GHaOHOHzGHOHrGHOHzGfi:

H3 H3 CH3 (6) 6-benzylamino-3,9-diethyltridecane having the formula G3H5 OIHS OHzOHgGH20mtinomomcflomomt lflcmcH,

in which R; and R are alkyl groups containing from 7 to 11 carbon atoms.

The stripping of the uranium-pregnant amine reagent is generally accomplished with an aqueous solution of an inorganic salt. In the example sodium chloride was employed, however, other inorganic salts such as sodium sulfate, sodium nitrate and the corresponding potassium and ammonium salts could likewise be used. These stripping solutions are generally acid and have a pH below 7.0. The solution may be sufficiently acid as to have a pH approaching 0.

While sodium sulfide is the preferred reducing agent from the standpoint of cost and ease of operation other reducing agents such as nascent hydrogen, hydrogen sulfide, sodium hydrosulfide, sodium thiosulfate, the alkali metal sulfides and other reducing agents can be used. As a practical matter only those reducing agents which 7 would not contaminate the organic phase or aqueous uranium product would generally be used. In general, the sulfur-containing reducing agents will not contaminate the product. In addition, the sulfur-containing reducing agents in which the sulfur has a valence of plus two or lower have been found to be particularly effective.

Stripping of molybdenum from the bleed stream of the amine reagent is preferably accomplished by means of the sodium carbonate solution. However, any alkaline solution having a pH in excess of 7 may be used. Such solutions may be the alkali metal hydroxides or carbonates or the alkaline earth metal hydroxide. It is merely important to have a high pH in order to strip the molybdenum from the bleed stream.

Now, therefore, we claim:

1. In a process for recovering uranium values from uranium solutions containing molybdenum values involving (1) the extraction of the uranium and molybdenum values with an amine reagent to produce an amine extract containing molybdenum and uranium values, and (2), the subsequent stripping of the uranium values from the amine reagent, the improvement which comprises preventing precipitation of a complex of molybdenum with the amine reagent by treating said molybdenum and uranium containing amine extract subsequent to said extraction and prior to said stripping of uranium values with a reducing agent selected from the group consisting of nascent hydrogen, hydrogen sulfide, sodium hydrosulfide, sodium thiosulfide and the alkali metal sulfides thereby preventing precipitation of said complex of molybdenum with said amine reagent.

2. A process according to claim 1 in which the uranium 8 is stripped from the extract with an aqueous solution of sodium chloride having a pH at least as low as 7.0.

3. Process according to claim 1 in which the amine reagent is an amine selected from the group consisting of secondary and tertiary amines.

4. Process according to claim 1 in which the amine reagent is a secondary amine containing alkyl substituents each containing from 8 to 22 carbon atoms.

5. Process according to claim 1 in which the amine reagent is a tertiary amine containing alkyl substituents each containing from 8 to 22 carbon atoms.

6. Process according to claim 1 in which said reducing agent is an alkali metal sulfide.

7. Process according to claim 1 in which the reducing agent is sodium sulfide.

8. Process according to claim 1 in which said reducing agent is sodium thiosulfate.

References Cited in the file of this patent UNITED STATES PATENTS Brown et al. Mar. 10, 1959 OTHER REFERENCES 

1. IN A PROCESS FOR RECOVERING URANIUM VALUES FROM URANIUM SOLUTIONS CONTAINING MOLYBDENUM VALUES INVOLVING (1) THE EXTRACTION OF THE URANIUM AND MOLYBDENUM VALUES WITH AN AMINE REAGENT TO PRODUCT AN AMINE EXTRACT CONTAINING MOLYBDENUM AND URANIUM VALUES, AND (2),THE SUBSEQUENT STRIPPING IF THE URANIUM VALUES, FROM THE AMINE REAGENT THE IMPROVEMENT WHICH COMPRISES PREVENTING PRECIPITATION OF A COMPLEX OF MOLYBDENUM WITH THE AMINE REAGENT BY TREATINGS SAIDMOLYBDENUMAND URANIUM CONTAINING AMINE EXTRACT SUBSEQUENT TO SAID EXTRACTION AND PRIOR TO SAID STRIPPING OF URANIUM VALUES 